There is known, as a device for measuring the distance between a host vehicle and the preceding vehicle, a device that emits laser light ahead of the host vehicle and receives reflected laser light produced when the laser light is reflected off the preceding vehicle. This device is called a laser radar as described in JP H09-159764 A (hereinafter referred to as Patent Document 1), or occasionally called as an optical ranging sensor. Hereinafter, the laser light emitted by the laser radar is referred to as irradiation light.
In Patent Document 1, a plurality of vehicles have laser radars mounted on the front and rear ends thereof. In the case where one of two vehicles having the laser radars mounted thereon is the preceding vehicle and the other is the following vehicle running just behind the vehicle, irradiation light emitted by the laser radar mounted on the front end of the following vehicle is reflected off the preceding vehicle, so that reflected light occurs. The reflected light is received by the laser radar mounted on the front end of the following vehicle. In addition, irradiation light emitted from the rear end of the preceding vehicle is received by the laser radar mounted on the front end of the following vehicle.
If the irradiation light emitted from the preceding vehicle is received by the laser radar mounted on the front end of the following vehicle and a distance to the object is measured with the irradiation light misrecognized as the reflected light, an incorrect distance measurement result is outputted.
Accordingly, in Patent Document 1, if the preceding vehicle and the following vehicle have a positional relationship where one vehicle receives the irradiation light from the other vehicle, the one vehicle emits the irradiation light and receives the reflected light during no emission of the irradiation light from the laser radar of the other vehicle. It is determined, based on the number of laser lights received after the emission of the irradiation light, whether the vehicles have the positional relationship where one vehicle receives the irradiation light from the other vehicle. In the case of receiving a plurality of laser lights after emitting only one irradiation light, it is determined that the vehicles have the positional relationship where one vehicle receives the irradiation light from the other vehicle.